I think this is likely a missing subordinate relation definition in your metadata.yaml file. Take a look at metadata.yaml - charm-ubuntu-advantage - [no description] as an example.
You have the subordinate: true definition, but you also need a relation that includes scope: container. If you’re not planning to pass any information over the relation you can use the implicit juju-info relation as we do here, or if you plan to pass information over the relation you’ll want to choose another relation name and interface name.
In terms of documentation about this I found Subordinate applications but I’m not sure where that’s exposed particularly.